During the developmentally crucial early and middle stages of organogenesis, normal morphogenesis requires low oxygen tensions. At these stages, the embryo represents a potential target of redox cyclers since redox cycling can deplete essential tissue oxygen which the hypoxic embryo cannot replenish. Additionally, oxygen depletion is conducive to the generation of reactive metabolites that can form adducts with critical macromolecules. Later, these agents pose a new threat since increased oxygenation promotes the generation of reactive oxygen species which the embryo is poorly equipped to detoxify. The mitochondria of embryonic tissues exhibit asynchronous functional maturation. Out preliminary studies indicate that this is a determinant of the pattern of embryonic tissue sensitivity to agents that either induce hypoxia or generate reactive oxygen species. It is possible to distinguish morphologically between dysmorphogenicity induced by hypoxia, oxidative species or macromolecular damage. The proposed studies will facilitate prediction of bioreducible agents that are embryotoxic, the nature of their dysmorphogenicity and the developmental stages and tissues that exhibit maximal sensitivity. We hypothesize that the embryotoxicity of a bioreducible agent can be predicted by (1) the relationship between its capacity to redox cycle versus its capacity to induce macromolecular damage in embryonic tissues, (2) the oxygenation at the time of exposure and (3) functional mitochondrial maturity of exposed tissues. To investigate this hypothesis, we proposed to: (1) Determine the capacity of a series of bioreducible agents to redox cycle in embryonic tissue; (2) Determine the capacity of the same agents to induce embryonic DNA damage; (3) Derive the ratio between 1 and 2 and determine its relationship to embryotoxic sensitivity; (4) Study the relationship between embryonic oxygenation and embryotoxic sensitivity to bioreducible chemicals; (5) Analyze the relationship between functional development of mitochondria and sensitivity of tissues to bioreducible chemicals.